“The bottom line is this: Our objective is clear, and that is to degrade and destroy (Islamic State) so that it’s no longer a threat not just to Iraq but also the region and to the United States," Obama said.

This statement jibes with the aggressive remarks Secretary of State John Kerry made after the killing of U.S. journalist James Foley by ISIS:

But later on in the news conference, the president stated a slightly different objective.

"We know that if we are joined by the international community, we can continue to shrink ISIL’s sphere of influence, its effectiveness, its financing, its military capabilities to the point where it is a manageable problem," Obama said.

The White House didn't immediately respond to a request for comment about the apparent contradiction.

Obama emphasized that degrading or destroying the militant group would take time because of the power vacuum in Syria, the abundance of battle hardened fighters who grew out of Al Qaeda in the Iraq war, and the need to build coalitions, including with local Sunni communities.

"This is not going to be a one-week or one-month or six-month proposition because of what’s happened in the vacuum of Syria, as well as the battle-hardened elements of (Islamic State) that grew out of Al Qaeda in Iraq during the course of the Iraq war ... it’s going to take time for us to be able to roll them back."

The White House said late on Tuesday that Obama was sending three top officials — Secretary of State John Kerry, Defense Secretary Chuck Hagel, and counterterrorism adviser Lisa Monaco — to the Middle East "in the near term to build a stronger regional partnership" against Islamic State militants.

"Whatever these murderers think they will achieve with killing innocent Americans like Steven, they have already failed," Obama said. "They failed because, like people around the world, Americans are repulsed by their barbarism. We will not be intimidated."

The U.S. resumed airstrikes in Iraq in August for the first time since the pullout of U.S. troops in 2011, and Obama said the strikes were already proving effective.

"Those that make the mistake of harming Americans will learn that we will not forget and that our reach is long and that justice will be served," he said.

(Reuters reporting by Balazs Koranyi and David Mardiste; Editing by Jeremy Gaunt)